Direct positive silver halide photographic materials are used for copy of various kinds of photographs. In particular, they have been frequently used in a photomechanical process in order to effect positive-positive or negative-negative printing of original half-tone images in the step called "contact work". In the contact work step, it is necessary for the photographic material to have good handling capabilities under a safelight. Various photographic materials which can be handled under orthochromatic and panchromatic safelights have so far been known. Under safelights of those wavelengths, however, there are disadvantages in that workability is low and production cost is high. Therefore, photographic materials which can be used under a visible safelight (or a safelight which can emit predominantly light of wavelengths longer than 450 nm) have been developed in recent years. (Photographic materials of this kind are called bright room-type photographic materials, hereinafter).
However, so far as direct positive silver halide photographic materials are concerned, ways of modifying them so as to be usable under daylight have not yet been developed. As a result, the graphic arts are still subject to such undesirable phenomena as lowering of maximum density, undesirable change in halftone dot area and so on.
As for the reversal characteristics of a direct positive silver halide emulsion, particularly D.sub.min thereof, those of silver chlorobromide are more desirable than those of silver chloride and further, those of silver bromide are preferable to those of silver chlorobromide. Accordingly, as the sensitive region is extended to the longer wavelength side, a large amount of dye must be used, which often causes various problems, for example, deterioration of reversal characteristics including decreases in contrast, increases in D.sub.min upon long-range storage, and so on.
Further, it is necessary for the photographic materials of the foregoing kind to be sufficiently fogged in order to attain low sensitivity, and insufficient fogging leads often to the problem of a rise in the minimum density (D.sub.min) of reversal.
Furthermore, even if low D.sub.min is attained immediately after production of the photographic materials, the photographic materials are still subject to undesirable phenomena from the view point of the photomechanical process, e.g., increase in D.sub.min, under conditions such as long-range storage, high temperature and high humidity, and so on.
Many attempts to improve D.sub.min of highly sensitive, direct positive silver halide emulsions have so far been made. However, they cannot be said to attain sufficient improvements, so further improvements are desired.
Therefore, the development of photosensitive materials which can lower D.sub.min of reversal in direct positive silver halide emulsions and do not suffer from an increase of D.sub.min upon long-range storage thereof, and further a method of making it possible to satisfy two contradictory requirements, one requirement being to lower D.sub.min, and the other requirement being to give a satisfactory safelight aptitude, are much wanted.